With shopsite, the customer never has to leave your site.
I don't know of any free carts that are super easy to customize...they all require a bit of work. Even zencart and oscommerce *can* be customized with a bit of work. The good thing is that once you've done the work once, you don't need to mess with it again, and it makes it a lot easier to add and manage products. You can then use HTML to add the shopping cart buttons to existing pages (but I would recommend using their cart pages).
You may want to check out this cart as well:
http://www.dansie.net/cart.html